Reasons for backfiring and erratic running...

[ClayW]

What are all the possible reasons for backfiring and erratic idling?

I've got a 2L with a Weber 32/36 and it's currently running with EDIS via Megasquirt. Lots of backfires and it just idles and runs like crap. Intuition tells me that I've got an ignition problem, though I've run EDIS for about three years now and I've never had a problem with it until now.

What we've done so far:

1. Unplugged the Megasquirt unit

Result: The problem persists with or without the Megasquirt computer hooked up. FYU, when you disconnect the MS unit, the EDIS defaults to 10 degrees BTDC. Erratic idle and backfiring either way. This leads me to believe that there's nothing wrong with the MS unit. Additionally, I've reloaded Windows on my PC and have yet to reload Megatune (also a little fuzzy on how). Being that the problem continues with or without the MS unit plugged in, I don't think that there's problem there.

2. Rewired the entire EDIS setup

Result: We thought that there might be some bad sheilding or something, but the new, much prettier wiring produced the same result.

3. Replaced the plugs, wires and coil pack

Result: No improvement. The old plugs weren't corroded or oily.

4. Inspected the 36-1 ring and VR sensor position

Result: No teeth are missing in the ring and the VR sensor is firmly in position and properly gapped.

Next to do: I've ordered a replacement EDIS module. Other than the VR sensor, that's the last thing to get replaced in the entire ignition system.

I doubt that it's a carb/fuel metering issue. is it possible that there's a valvetrain issue? Where should I be looking and for what? Maybe a chain tensioning issue? Any ideas?

Thanks in advance.

[Mike87]

TIMING!

[Lee]

Backfiring is usually caused by running too lean. A lean mixture takes longer to ignite and burn... often resulting in mixture still burning when valves open...

Lee

[ClayW]

Backfiring is usually caused by running too lean. A lean mixture takes longer to ignite and burn... often resulting in mixture still burning when valves open...

Lee

Does that mean that I might have a clogged jet in the carb? The filter does have some crap in it from the bottom of the tank. It's not beyond reason to think that something might have made it into the carb.

[nbristow01]

I tuned your car? that would do it

[Swiftus]

Going to check carb jets right now... And if that shows nothing I'll pull the valve cover and snap a photo or two.

Cross your fingers!!!

[Lee]

Going to check carb jets right now... And if that shows nothing I'll pull the valve cover and snap a photo or two.

Cross your fingers!!!

Also, if the ignition doesn't have enough advance (like missing a lot of advance), the engine can also backfire.

[Swiftus]

Cleaned up the carbs jets. I didn't dismantle the carb and clean in entirely because I don't have a new set of gaskets. However, I did spray carb cleaner through the jets (out of the carb) and cleaned up what I could see that was exposed. Added new clean fuel filters. Car is still missing erratically, but there is no longer a backfire at any rev range either on just the EDIS or the Megasquirt.

While in EDIS-only mode, there is a miss every 4 to 6 rotations (buh buh buh buh buh buh buh blaaauhhhh blaaauhhh buh buh buh buh buh buh buh buh buh buh blaaauuhhhh buh buh blaaaauuuhhh etc...)

When in Megasquirt, there is a miss every other rotation or so... (buh buh buh blaaauuhhh buh blaauuhhhh buh buh buh buh buh blaaauuuhhh etc...)

So cleaning up the fuel delivery did work a bit.

[c.d.iesel]

broken rocker arm

broken/loose valve adjusting ecentric

compression test

blown head gasket

valve adjustments

and what's with all the computer stuff with a carburetor ?

what am i missing here?

[Mike87]

When my car started doing something similar (I have Electromotive crank-fire ignition) it turned out to be the magnetic pickup was failing. A replacement part fixed it right up.

[Swiftus]

Good to know. I just wish that I had a scope so that I could see the signal coming from the VR sensor. I think that Clay may be getting another VR sensor with his other EDIS, so we can see if that fixes the issue.

Well this is an excuse to go and get a legit scope...hehehe

[kcdonahue]

If you guys need another VR sensor and edis box I can bring one done on Wed... I have to go to the Dr. tomorrow but will be back in the hood on Wed morning.

KC

[JohnN]

If not, there is an excellent article in the current MegaManual about the tooth size being mismatched to the VR sensor.

If you're running the MS-II processor, there is a tooth analyzer program that may help: http://www.microsquirt.info/tachref.htm

Good luck!

John N

[JohnN]

(nt)

[ClayW]

If you guys need another VR sensor and edis box I can bring one done on Wed... I have to go to the Dr. tomorrow but will be back in the hood on Wed morning.

KC

I have one of each on the way. After hearing it run today (after Jay cleaned the jets and the bowl), I think that it's all in the carb. It really sounds a LOT better after just a little cleaning. A full carb rebuild is in order.